In radio beacon-based road toll systems, e.g. according to the dedicated short-range communication (DSRC) or wireless access in a vehicle environment (WAVE) standard, onboard units (OBUs) carried by the vehicles communicate with geographically distributed radio beacons via short-range radio transmission as soon as they pass these. The radio communication generally serves to locate the vehicle on the radio coverage area of the radio beacon to charge for usage of locations or also simply to transmit toll data generated by the OBU to radio beacons on its route.
It is often desirable to determine the distance at which a vehicle passes a radio beacon, e.g. to penalise toll violations in the case of multi-lane roads. When multiple vehicles travelling next to one another in different lanes pass the radio beacon and one of its radio communications indicates atoll violation, e.g. a missed toll charge debit, an inadequate balance of a charge account, a defective or incorrectly adjusted OBU etc., or a lane-dependent charge rate or toll (multiple occupant lane) is to be calculated, then it is essential to know which of the vehicles travelling next to one another is responsible for the toll violation. The toll violation may be identified, e.g. visually in situ or by an evidence photo of the road section of the beacon.
Various methods of determining the distance are currently known. One solution is to use multiple physically offset receiving antennae in the radio beacon to determine the positions of the OBUs in the radio receiving field from phase difference measurements between the OBU signals received by the individual antennae. Another solution is known from the U.S. Pat. No. 5,790,052 and is based on Doppler measurements of the different relative speeds of an OBU in relation to physically offset receiving antennae of a radio beacon to determine the ratio of the distances from the two receiving antennae from the ratio of the speed measured values. Also, it would is possible to use a separate radio beacon with a low radio coverage range for each lane. All these known solutions are expensive, because they are based on multiple receiving antennae.